iris: Don't check other batches for our batch BO

iris: Don't check other batches for our batch BO

This is an awkward corner case.  We create batches in order, each of
which creates and pins a BO.  The other batches may not be set up yet,
so it may not be safe to ask whether they reference a BO.

Just avoid this for now.  We could avoid it for other context-local BOs
too, but we currently don't have a flag for that (and I'm not certain
whether it's worth it).